1. Field of Invention
The present invention relates to a semiconductor light emitting device including a photonic crystal.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region.
The quality of an LED can be characterized by, for example, its radiance, which is the power emitted in a specified direction per unit area of an emitting device, and its extraction efficiency, which is the ratio of photons extracted from the device to photons generated in the light emitting region. The extraction efficiency is limited by, among other things, the emitted photons suffering multiple total internal reflections at the walls of the high refractive index semiconductor crystal that forms the p-type, n-type, and light emitting regions of the device. As a result, many of the emitted photons do not escape into free space, leading to poor extraction efficiencies, typically less than 30%.
U.S. Pat. No. 5,955,749, entitled “Light Emitting Device Utilizing a Periodic Dielectric Structure,” granted to J. Joannopoulos et al., describes the use of a photonic crystal to enhance the extraction efficiency. A photonic crystal is created by forming a lattice of holes through the semiconductor layers of the light emitting diode. The lattice of holes creates a medium with a periodically modulated dielectric constant, affecting the way light propagates through the medium. If the proper lattice spacing is chosen, light that would otherwise have been trapped in the structure by total internal reflection can now escape, increasing the extraction efficiency of the LED. Needed in the art are efficient photonic crystal designs.